1. Field of the Invention
The present invention relates to electrical contactors and similar devices for completing and interrupting electrical current-carrying paths between a source of electrical energy and a load. More particularly, the invention relates to a coil assembly and actuator for such a device which facilitates assembly and installation, and which provides improved electrical, magnetic and thermal performance during transient and steady state phases of operation.
2. Description of the Related Art
A great variety of devices have been designed for completing and interrupting current-carrying paths between an electrical source and an electrical load. In one type of device, commonly referred to as a contactor, a set of movable contacts is displaced relative to a set of stationary contacts, so as to selectively complete a conductive path between the stationary contacts. In remote-controllable contactors of this type, an actuating assembly is provided to cause the movable contacts to shift between their open and closed positions. Such actuating assemblies typically include a coil forming an electromagnet, and a core to intensify a magnetic field generated around the coil when an actuating current is passed therethrough. The magnetic field attracts a movable armature which is coupled to the movable contacts within the device, thereby displacing the movable contacts and thus making electrical contact or closing the electrical circuit. When the actuating current is removed, biasing members return the movable assembly back to its normal position thus breaking the electrical connection or opening the electrical circuit.
Contactors of the type described above are commonly available with either alternating current or direct current actuating coil assemblies. The selection of either an alternating current assembly or a direct current assembly typically depends upon the type of electrical power available in the application. However, advantages and disadvantages are associated with each type of assembly. For example, direct current coils can be associated with simple solid core structures which do not need to minimize heating from circulating eddy currents found in alternating current coils. Also, direct current coils tend to have a higher force to power ratio because the current is steady and does not pass through zero with each half cycle as is the case with alternating current, and therefore require lower currents to obtain a desired armature pull-in or contact retaining force. Moreover, direct current assemblies do not require shading coils as are typically provided in alternating current assemblies, and therefore are quieter in operation and experience lower wear. On the other hand, alternating current power sources are very widespread and are favored in many cases due to their availability.
Coil assemblies for contactors have also been constructed with multiple coils, including coaxially aligned pickup coils and holding coils. Because a greater coil MMF is often required to close the contactor than is required during steady-state operation (i.e., after closure), both the pickup and holding coils are energized during closure, and with the pickup coil being deenergized following closure. The pickup coil is designed to have a significantly higher MMF and power than the hold coil. Turning off the pickup coil minimizes heating and reduces the power required once the armature has closed (i.e. steady state operation). Timing for deenergization of the pickup coil is typically fixed, and is set so as to provide sufficient force and time for displacement of the movable contact assembly to a closed position. However, if the time or force varies, as is sometimes the case, such arrangements may either provide insufficient or excessive periods of energization of the pickup coil. Also, such devices typically employ mechanical switches to release the pickup coil, or to switch the pickup coil in series with the holding coil following the initial closure period.
In addition to the foregoing drawbacks, where conventional coil assemblies are associated with control circuits supported on conventional circuit boards, these must often be supported by additional structures in the coil assembly or in the housing adjacent to the coil assembly. These structures add further to the cost of the device, and require additional labor for installation. Moreover, in multiple-coil actuating assemblies, care must be taken to ensure that proper polarity of the pickup and holding coils during electrical connection to the control circuit. Again, this can add to the cost of the device, and, in the event of an error in the polarity of the connections, can result in malfunction or the need to rework the assembly.
There is a need, therefore, for improved operator structures for contactors and similar electrical devices. In particular, there is a need for an actuating coil assembly in which multiple coils can be provided to reduce the power to the device during steady-state operation, but in which a pickup coil is energized for sufficient time to ensure adequate movement of the movable contact assembly. There is also a need for an improved coil structure which facilitates mounting of control circuit components and wiring of coil leads, thereby facilitating manufacturing of the overall assembly.
The invention provides a novel approach to the design of contactor actuating coil assemblies and the control of assemblies designed to respond to these needs. The technique employs a dual-coil assembly including a pickup coil and a holding coil. Both coils may be energized for actuation of the device. The pickup coil is then deenergized based upon an input signal which is derived from a sensed parameter of the energization signal, such as voltage. The pickup coil is thus energized for a sufficient time to ensure closure of the movable elements in the device. The holding coil may be powered by direct current which is produced by a rectifying circuit when the incoming power to the device is an AC wave form. The holding coil current is rapidly dissipated by control circuit upon deenergization of the main coil terminals, thereby avoiding the creation of induced currents and associated magnetic fields upon release of the device. The coil may then benefit from all of the advantages from a DC coil structure, while offering the advantage of being powered by an AC power source. The coil structure also provides a simple and convenient arrangement for supporting a control circuit board on a coil subassembly. The coil subassembly also facilitates proper wiring of the pickup and holding coils to the control circuit board. In a preferred configuration, common leads are brought from the coil assembly in a central location, thereby facilitating identification of the leads for electrical connection to a circuit board.
Thus, in accordance with a first aspect of the invention, an electromagnetic operator is provided for an electrical contactor. The operator includes a coil assembly, including a first coil and a second coil. A first switching circuit is coupled to the first coil and is configured to apply energizing current to the first coil in response to a control signal. A second switching circuit is coupled to the second coil and is configured to apply energizing current to the second coil in response to the control signal for a variable duration which is a function of a parameter of the control signal. The second switching circuit may apply the energizing current to the second coil for a duration which is based upon the voltage of the control signal. Moreover, the second switching circuit may include an analog timing circuit which interrupts power to the second coil after the variable duration.
A common support may be provided for both coils, and the coils may be wound coaxially on the support. Flanges extending from the support may serve to mechanically support the first and second switching circuits. Leads directed to the switching circuits may be channeled through guides in the support. Moreover, the coil assembly may include a magnetic base support defining a core or armature of the assembly.
In accordance with another aspect of the invention, a control circuit is provided for an electromagnetic operator. The operator includes first and second coils for generating actuating fields in response to energizing signals. The control circuit includes a first switching circuit coupled to the first coil and configured to apply a first energizing signal to the first coil. A second switching circuit is coupled to the second coil and is configured to apply a second energizing signal to the second coil for a variable duration after application of the first energizing signal to the first coil. The second switching circuit may include a timing circuit wherein the variable duration of application of the second energizing signal is determined by the configuration of the timing circuit. The first and second switching circuits may be coupled across a common direct current bus, and the first and second energizing signals may be applied by the direct current bus.
In accordance with a further aspect of the invention, a coil assembly is provided for an electromagnetic operator. The coil assembly includes a coil support having first and second annular recesses defined between upper and lower flanges, and separated from one another by a central flange. First and second lead guides are defined in the central flange. A first coil is wound in the first annular recess and has a first lead disposed in the first lead guide. A second coil is wound in the second annular recess and has a second coil disposed in the second lead guide. A control circuit board may be supported on the coil support and coupled to the leads.
In accordance with a further aspect of the invention, a method is provided for actuating an electrical contactor. A contactor includes an electromagnetic operator, a carrier displaceable under the influence of the operator, stationary contacts, and movable contacts, movable by the carrier to selectively contact the stationary contacts. In the method, an energizing signal is first applied to the first and second coils in the operator to energize the first and second coils. The energizing signal is then removed from the second coil a variable period of time after application of the energizing signal to the second coil. In a particularly preferred embodiment, the variable period of time is a function of a parameter of the energizing signal, such as voltage
In accordance with a further aspect of the invention, a flat plate armature is utilized to provide reduced mass and lower return spring force resulting in low magnetic pickup force requirements and hence low coil power requirements. Furthermore, the armature has a thin cross section which saturates at small air gaps thereby reducing velocity and impact force upon closure. Additionally, this construction facilitates greater acceleration upon opening due to the decreased mass of the armature.